Wake turbulence is a product of aircraft take-off and landing, the strength of which is governed by the weight, speed, and wing-span of the aircraft. Wake turbulence may also be effected by environmental conditions. A following aircraft needs to avoid the wake turbulence of the leading aircraft for safety, so runway take-offs and landings are spaced apart to allow time for the strength of the wake turbulence of a leading aircraft to decay to a safe level. The spacing required for safe runway usage is a limiting factor in airport capacity and some airports merely use a set time frame for all following aircraft regardless of other factors.